1. Field of the Invention
This invention relates to a light-emitting device, and particularly to a light-emitting device suitable for use in an image forming apparatus such as a copying apparatus, a facsimile apparatus or a printer.
2. Related Background Art
An LED recording head is comprised of a number of LED chips and a drive IC for driving them linearly mounted on a substrate, and when it is used, for example, in a copying apparatus, light is applied from an LED element onto a photosensitive drum to thereby form a latent image.
FIGS. 1A and 1B of the accompanying drawings are a schematic circuit diagram and a cross-sectional view, respectively, showing the construction of an LED head accord ing to the prior art. LED elements 1-1, 1-2, . . . , 1-5 linearly arranged in an LED chip 1 are connected to transistors 4-1, 4-2, . . . , 4-5 in a drive IC 10 in one-to-one relationship by bonding wires 2-1, 2-2, . . . , 2-5. Limiting resistances 3-1, 3-2, 3-5 prescribe electric currents for driving the LED elements 1-1, 1-2, . . . , 1-5 they take under their charge. In the drive IC 10, there are provided the limiting resistances 3-1, 3-2, . . . , 3-5, the transistors 4-1, 4-2, . . . , 4-5 and a control section 5. The LED chip 1 and the drive IC 10 are disposed on one and the same substrate 6.
Now, it is known that an LED is generally reduced by about 1% in its light emission intensity for a one-degree rise of environmental temperature, and to effect image reproduction of high quality, it is important to suppress a temperature rise including that of the drive IC 10. When the fluctuation of this output is great, the application is limited to only binary recording.
In the prior-art LED recording head of FIGS. 1A and 1B, generally the limiting resistances 3-1, 3-2, . . . , 3-5 substantially determine the temperature rise of the drive IC 10 including the control section 5. Heat generated by these limiting resistances is transmitted to the LED elements via the bonding wires 2, thus resulting in a reduced light emission output.
The limiting resistances are once dropped onto the substrate by the bonding wires and further are connected from the substrate to the LED chip by the bonding wires, whereby the heat is discharged to the substrate and the heat conducting action is alleviated, but the number of bonding wires becomes double, and when a recording head of high resolution and great length is to be realized, it is unrealistic in terms of cost.
Also, to place only these limiting resistances out of the drive IC 10 and dispose them at a location thermally remote from the LED chip 1, thousands of chip resistances must be mounted discretely from the drive IC and thus, the substrate itself becomes bulky, and this also is unrealistic.